Relative Fat Mass (RFM) is a simple, formula-based estimate of whole-body fat percentage that only needs your height and waist circumference. It was developed as a practical alternative to BMI, which cannot distinguish muscle mass from fat. This calculator walks through the formula, what each input means, and where the estimate's limits are.
How the Relative Fat Mass Calculator works
RFM was introduced by Woolcott and Bergman in a 2018 study published in Scientific Reports, which regressed height-to-waist ratio against body fat percentage measured by dual-energy X-ray absorptiometry (DXA) in a large sample of American adults. The resulting formulas — RFM = 64 minus 20 times height divided by waist for men, and RFM = 76 minus 20 times height divided by waist for women — use a higher base constant for women because women typically carry a higher healthy percentage of body fat than men at the same relative waist size. Because the formula uses a ratio, height and waist must be entered in the same unit (both centimeters or both inches); no unit-conversion factor appears in the equation itself.
Inputs and what they mean
Height should be measured standing, without shoes. Waist circumference should be measured around the narrowest point of the torso, typically level with the navel, with the tape snug but not compressing the skin — do not measure after exhaling forcefully or "sucking in." Of the two inputs, waist circumference has by far the larger effect on the result: a few centimeters of difference in waist size shifts RFM noticeably, while height changes have a smaller, inverse effect. Selecting the correct sex is essential, since the formula's base constant differs by 12 points between men and women.
Limits and edge cases
RFM is a population-level regression estimate, not a direct measurement — it will not match a DXA scan, bioelectrical impedance reading, or skinfold measurement exactly for any individual, and accuracy can be reduced for very short, very tall, or very muscular people, as well as pregnant individuals. It also does not account for age, which is known to influence body-fat distribution independently of waist size. Treat the result as a screening estimate to track over time or discuss with a healthcare professional, not a diagnostic measurement, and pair it with other health indicators rather than relying on RFM alone.